Conc.Sulphuric acid or concentrated Phosphoric acid

Alcohols are amphoteric. So, the lone pair of oxygen atoms makes the -OH group weakly basic. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion ⁺OH2.This basic characteristic of alcohol necessarily helps in conversion to propene when dehydrated with a strong acid.